Semiconductor devices are typically fabricated by attaching the device, e.g. a light emitting element or laser, to a mount or header. A header comprises a base plate with two major surfaces and a heatsink affixed to one of the major surfaces. The heatsink is usually of a good thermal conductor-like copper and has a device mounting area to which the device can be soldered or bonded. The header may also include a stud, pins, or the like on the other major surface suitable for mounting the header onto an external support.
It is known to metallize the header, e.g. with nickel and gold, prior to bonding the light emitting element thereto. Electroless nickel is widely used because of the excellent coverage it is known to provide on the intricately shaped headers. Further, deposition of electroless nickel is convenient on a variety of base metals with minimal surface preparation. The nickel layer acts as a barrier to prevent any diffusion of copper into the light emitting device. The electroless nickel layer is usually followed with a layer of gold. The gold is a protective layer with good conductivity which also enhances solderability of the device.
There are problems, however, in using the electroless nickel for the above application. First, the most convenient-to-deposit forms of electroless nickel contain a certain amount of phosphorus. When the mounting area of the copper heatsink is heated during the process of soldering the light emitting device thereto, the phosphorus migrates to the surface forming detrimental intermetallic compounds with the solder. These intermetallic compounds can adversely affect the thermal and electrical conductivity, as well as the physical strength, of the solder bond.
Also, it is desirable to have a sharp edge on the copper heatsink terminating the mounting area. Because of its excellent coverage characteristics, an electroless nickel layer tends to be somewhat nonconformal thereby rounding off and compromising the sharpness of the edge of the heatsink.
It is known that the nickel deposited by an electrolytic process is more conformal and of the highest purity. Unfortunately, the electrolytic nickel solution does not "throw" into all of the contours and crevices of the header as well as the electroless nickel solution does. Therefore, it is harder to get an overall coverage on the header using electrolytic nickel. Further, the copper heatsink should be treated in a nitric-sulfuric etch solution preparatory to deposition with electrolytic nickel. This solution is known to attack the steel base plate and stud of the header.
Attempts to selectively etch the copper by masking the steel parts with waxes, photoresists and the like have proven to be impractical for production purposes. Applying, delineating, and removing mask materials from large numbers of headers is a highly tedious task. Further, many masking materials leave an organic residue on the metal parts detrimental to subsequent plating.
It would be desirable therefore, to have a device header which could combine the advantages provided by the electroless and electrolytic nickel processes and a method for fabricating such a header.